Actin and actin-binding proteins I. The fundamental analysis

Actin is one of the most abundant and highly conserved proteins on earth. I ts transient polymerization and depolymerization are essential during chang es in cell shape and locomotion. In most eucaryotic cell types, actin is re presented either in a monomeric (G actin) or in a polymeric form (F-actin). The :filament turnover cycle occurs in vitro (with pure actin) and in vivo . Subsequently, it is thought to proceed through the addition of an ATP-mon omer to the barbed end of the filament, the hydrolysis of ATP, the release of Pi, and the dissociation of the ADP-monomer from the pointed end. The ra te-limiting step in actin polymerization is nucleation, which is the assemb ly of the first subunit to generate a new filament. The plus (barbed) end g rows five to ten times faster than the minus (pointed) end. Actin has an ac tive role in many cellular functions and is thought to be involved as both a target and mediator of signal transduction through the Rho family of smal l GTPases. It is also dynamically regulated by a complex army of proteins c alled Acting-binding proteins.